Without knowing more about the NAS device and the driver I only have one idea that you can try.
Please try to enable the "Quick removal" feature for your network drive.
Click Start Right click on Computer » Manage » Device Manager Find your Network Drive in Disk drivers on the right side Right click on it, then select Properties » Policies panel Make sure that Quick removal (default) is selected.
The Quick removal option being enabled for a device should make it write the cache immediately and in the event of a removal may make it more resilient. This may not enable the ability to disconnect the drive if the driver does not support removal but at least the caching should be honored from the OS.
I hope this helps and please let us know the outcome.